Introduction & Epidemiology
Keeping cats indoors has become common veterinary advice to reduce the risk of exposure to infectious diseases and injury
from vehicles or other animals. This advice may not be completely beneficial for cats, however. As early as 1925, Kirk suggested
that "too close confinement to the house" increased the risk of lower urinary tract signs (LUTS). Results of subsequent epidemiological
studies have confirmed his observations. Available epidemiological studies of LUTS suggest an overall incidence rate of somewhat
less than 1%, and a prevalence rate from 1 to 6%; a recent study reported a rate of approximately 1.5%. Many factors associated
with LUTS have been investigated. Excessive body weight and decreased activity were associated with increased risk in some
studies, and cats that only had access to indoor litter pans had an increased risk of LUTS compared to cats that could eliminate
outdoors. Living with other cats also has been associated with LUTS, suggesting that social interactions or a horizontally
transmitted infectious agent might play a role in the development of these signs. The lack of difference between cases and
controls in viral disease rates, and the increase in risk associated with the amount of time spent indoors seems to argue
against an infectious agent as a common cause. Subsequent studies have found indoor housing to be associated with a variety
of common disease problems of cats, including dental disease, obesity, type 2 diabetes mellitus, and hyperthyroidism. In
addition to the factors listed above, a case-control study of LUTS conducted in New Zealand during 1991-1993 also reported
an increased incidence of LUTS after moving to a new house within the previous three months, and during winter months; further
analysis revealed a highly significant association with rainy days during the previous month rather than with season. Access
to outdoor prey was found to be protective.
Dental disease is reported to be the most common current disease problem of pet cats. The two most common problems are periodontal
disease and odontoclastic resorptive lesions (ORL). In 1992, Van Wessum et al. reported that ORL were present in 62% of Dutch
cats and 67% of US cats examined. Two cross-sectional studies have evaluated potential risk factors for ORL in feline teeth.
In the first, cats with ORL were more likely to be older, female, taking medications, and drinking city vs. well water, and
less likely to play with toys, have owners who cleaned their teeth or to be fed diets with higher magnesium, calcium, phosphorus,
and potassium contents. Without food intake information, the significance of the differing mineral content of the diets is
unclear, although it may suggest that diets designed to influence recurrence of LUTS were fed. Indoor housing was not identified
as a significant risk factor in this study. In a subsequent study, an odds ratio (OR) of 4.5 was associated with a history
of dental disease (gingivitis, calculus, or periodontal disease), and ORs of 4.4 and 4.5 were found for city residence and
indoor housing, respectively. Consumption of commercial treats appeared protective (OR=0.3).
Obesity in cats also is a common problem. In a study of the body condition score of more than 2000 cats presented to veterinary
hospitals in the Northeastern United States in the early 1990s, veterinarians reported that 25% of cats were overweight, and
owners estimated that 29% of their pets were overweight. Apartment dwelling, inactivity, middle age, being male, neutered,
of mixed breeding, and certain dietary factors were associated with being overweight. The OR for indoor housing ranged from
1.6 to 15.8, depending on the parameter measured. Another study of obesity in cats living in metropolitan Perth, Western
Australia, in which cats were categorized as underweight, correct-weight or overweight by their owners found an OR of 1.4
for indoor housing and 1.8 for living in houses with only one or two other cats. The ORs for neutered and crossbred cats
were 2.8 and 2.1, respectively. In a recently reported study of obesity in an urban cat population in New Zealand, inactivity
was identified as significant following univariate analysis, but not in the combined logistic-regression analysis. Scarlett,
et al., suggested that the increased risk of obesity in apartment-dwelling cats might have been due to inactivity and boredom.
Additionally, Carlstead, et al., reported that passivity and reduced activity can occur in environments containing threats
perceived to be inescapable or uncontrollable.
Prior to 1979, hyperthyroidism in cats was a rare condition. A 1988 study identified an OR for indoor housing of 4 to 11.2;
Siamese cats were found to be protected. In a subsequent study, a reduced risk also was identified in Siamese and Himalayan
cats, and a 2- to 3-fold increase in risk was observed in cats eating a diet composed mostly of canned cat food and in those
using cat litter. More recently, cats that consumed fish or liver and giblets flavors of canned cat food were at increased
Most of these studies, conducted over a period of more than a quarter century, have identified indoor housing as a risk factor
for disease in cats. Differences among the studies, particularly those that did not find increased risk, might have occurred
for a variety of reasons. The first, of course, is that indoor risk really is not a risk factor. The fact that it has been
identified in different diseases studied at different times and different places, however, seems to argue against this interpretation.
Differences in sample size and the questions asked undoubtedly also contributed to the differences. Additionally, the studies
that did not identify associations with indoor restriction were the most recently conducted. If the majority of cats in both
case and control groups (which in many studies were cats with other diseases) were housed indoor, it would be difficult to
isolate this as a risk factor.
The identification of differences in breed susceptibilities also suggests that internal as well as external factors influence
risk. Internal factors include temperament, genetic and experiential variables. As in other species, individual differences
in temperament have been reported in cats. Also, strain differences in stress susceptibility, and individual variation in
experience influence responses to the environment.